It is known in the field of micro and minicomputers to connect different computers or central processing units by means of data and address buses. In the case where two or more microprocessors share a bus, it is necessary to have some arbitration scheme to allocate control of the bus at any particular time. It is also known to make references within a single board microcomputer and also to make external references on a shared bus off that board. One commercial example is that of the Intel single board computer, in which each single board computer can operate on-board and is able to request a shared bus whenever it needs to make an access to other single board computers.
A problem with these devices is that it is necessary to indicate in some way whether a memory address is on-board or off-board and this indication takes computer code and time in the execution of the instructions.